Understanding SPF

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Christopher Miller, MD, Assistant Professor of Dermatology at the Abramson Cancer Center of the University of Pennsylvania, responds:

SPF numbers can be confusing. "SPF" stands for Sun Protection Factor. If the sunscreen is applied perfectly and it stays where you put it, the SPF tells you how long that your skin will be protected from burning. For example, if it normally takes 10 minutes for your skin to burn, then applying a sunscreen with an SPF of 15 will protect your skin for 15 times longer, or 150 minutes.

A higher SPF of 30 is not twice as good as an SPF of 15, however. A sunscreen with an SPF of 15 blocks 93% of the ultraviolet B rays. A sunscreen with an SPF of 30 blocks 98% of the ultraviolet B rays. There is only about a 5% difference in effectiveness between the two SPF values.

In addition, since sunscreen does not stay where you put it (especially if you're swimming, sweating, or wiping your skin), it's protection does not last for the full time. For example, if your skin would burn in 10 minutes, a sunscreen with an SPF of 30 will not protect your skin for 300 minutes. You need to reapply sunscreen every 2 hours, which is probably more frequently than the SPF rating suggests.

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